How should the size of from_io's "sliding buffer" be chosen?

[smoelius]

My question concerns the second component of from_io's val argument`:

postcard/src/de/mod.rs

Line 85 in 6338601

pub fn from_io<'a, T, R>(val: (R, &'a mut [u8])) -> Result<(T, (R, &'a mut [u8]))>

From what I can tell, it is a "sliding buffer":

• postcard/src/de/mod.rs

  Line 90 in 6338601

  let flavor = flavors::io::io::IOReader::new(val.0, val.1);

• postcard/src/de/flavors.rs

  Lines 330 to 335 in 6338601

  	pub(crate) fn new(reader: T, buff: &'de mut [u8]) -> Self {
  	Self {
  	reader,
  	buff: SlidingBuffer::new(buff),
  	}
  	}

In the loopback test, the buffer is 2048 bytes:

postcard/tests/loopback.rs

Lines 191 to 193 in 6338601

	let mut buff = [0; 2048];
	let x = ser.clone();
	let deserialized: T = from_io((x.as_slice(), &mut buff)).unwrap().0;

Is this a good choice, generally?

Thank you.

cc: @xgroleau

[xgroleau]

Hey @smoelius,

You just need to make sure that the buffer is big enough for all the data that is borrowed.
(i.e., if you have field: &'a T such as field: &'a str in your struct).
The borrowed data will be stored in that buffer, so it must fit in it.
Thus it depends on the format of the data, if everything is owned or implements DeserializeOwned, the buffer can be of length 0 actually.

The test shown is actually pretty overkill to use a buffer of 2048, nothing is actually borrowed in BasicU8S.
The test would pass if the buffer of lenght 0 such as let mut buff = [0; 0];.

For an example with borrowing, if instead we wanted to deserialize

#[derive(Debug, Serialize, Deserialize, Eq, PartialEq)]
struct BorrwedU8S<'a> {
    st: u16,
    ei: u8,
    sf: u64,
    tt: u32,
    borrowed: &'a u8,
}

We would need a buffer of at least a length of 1 to store the borrowed: &'a u8 field.

Hopefully that clarifies your question.

[smoelius]

Thank you, @xgroleau. Your response is very helpful. But should this example then work?

use postcard::{from_io, to_io};
use serde::de::DeserializeOwned;

fn main() {
    fn sanity(_: &impl DeserializeOwned) {}

    let data = String::from("hello");

    sanity(&data);

    let serialized: ::std::vec::Vec<u8> = vec![];
    let ser = to_io(&data, serialized).unwrap();
    {
        let mut buff = [0; 0]; // <-- Changed 2048 to 0.
        let x = ser.clone();
        let deserialized: String = from_io((x.as_slice(), &mut buff)).unwrap().0;
        assert_eq!(data, deserialized);
    }
}

I get:

thread 'main' panicked at src/main.rs:16:71:
called `Result::unwrap()` on an `Err` value: DeserializeUnexpectedEnd

I'm wondering if there may be a bug here:

postcard/src/de/deserializer.rs

Lines 387 to 392 in 6338601

	fn deserialize_string<V>(self, visitor: V) -> Result<V::Value>
	where
	V: Visitor<'de>,
	{
	self.deserialize_str(visitor)
	}

[xgroleau]

You are right, but it seems strings and probably deserialize_byte_buf and maybe others borrow the data first and then allocate. So in the end, that includes allocation actually, not just borrowed data. I misunderstood and didn't verify with the internals, sorry about that. Not sure if it is a bug more than just a misunderstanding of the actual behavior.

@jamesmunns could validate that if it is the actual desired behavior. If it is, then you need to make sure to have enough space for allocated items and we could probably document it to make sure the behavior is clear.